1. Field of the Invention
The present invention relates to a technique of attaching RFID (Radio Frequency IDentification) tags to transfer devices for transferring objects.
2. Description of the Related Art
In assembly lines or inspection lines for transferring components or products, it is common for the components or products to be assembled or inspected while being transferred on pallets or carts. When components are to be assembled or products are to be inspected while being transferred on transfer devices such as pallets, carts, or the like, bar code labels are often applied to the products or to transfer devices to be used for identifying the respective products or understanding the progress of respective steps such as assembling or inspecting, and thereby the bar codes are read with a bar code reader in order to collect necessary information. If it is desired that a bar code reader read information from bar code labels, then limitations on reading operations performed by operators, limitations whereby the positional relationship between the bar code labels and the bar code reader have to be constant, or other such limitations must be imposed.
RFID tags can hold more information than bar code labels and can store data. Because of this, RFID tags have been widely used recently for controlling the steps of assembly and the inspection of components and products.
When RFID tags are used for controlling assembly steps or inspection steps, RFID tags are conventionally attached to transfer devices such as pallets, carts or the like used for transferring products. Thereafter, necessary information is read and written from and to the RFID tags by using an antenna for an RFID reader/writer provided near the assembly line or the inspection line.
FIGS. 1 and 2 respectively show conventional methods of attaching RFID tags to transfer devices. FIG. 1 shows an example of attaching an RFID tag to a transfer pallet in a conventional method. An RFID tag 104 is affixed to the top surface or a side surface of a transfer pallet 100 that is for carrying transferred objects 103 such as components, products or the like. The spot to which the RFID tag 104 is affixed is determined in accordance with its positional relationship with an antenna attached to a reader/writer. Specifically, the RFID tag is affixed within the communication distance in such a manner that the surface of the RFID tag, from which data can be read highly accurately, faces the antenna while taking into consideration the communication distance and communication directivity.
Conventionally, in order to attach RFID tags to transfer carts, RFID tags are either affixed to particular spots or, when transferred objects are larger than the areas of the top surfaces of the carts, held in dedicated pockets provided in transfer devices.
FIG. 2 shows a conventional example of attaching an RFID tag to a transfer cart. A cart 110 carrying the transferred object 103 moves on a transfer rail (not shown). An antenna 101 for a reader/writer is arranged on the near side with respect to the cart 110 in FIG. 2.
When an RFID tag 104A is to be affixed directly to a component/product with adhesive tape or the like, or when an RFID tag 104B is to be held in a pocket 105 attached to the cart 110, the RFID tags 104A and 104B have to be attached while taking into account the distance and position that allow for the communication between the RFID tags and the antenna 101, 102 for the reader/writer.
Specifically, both in the case when the RFID tag is to be affixed directly to the transferred object 103 and in the case when the RFID tag is to be held in the pocket 105 attached to the cart, the tags 104A and 104B have to be positioned within the distance over which communication with the antenna 101 or 102 is allowed. The side including the RFID tag may not be closest to the antenna 101 or 102, depending on the direction of movement of the cart 110. Also, the surface of the RFID tag, from which data can be read highly accurately, sometimes may not be facing the antenna or the position at which the RFID tag is attached may be out of the range over which the communication is allowed, so that the accuracy of reading information from RFID tags is affected.
Because methods of attaching RFID tags to carts as described above are employed conventionally, the direction of movement of the carts is limited in accordance with the attached RFID tags when the RFID tags are used for performing production management or quality management in assembly/inspection lines. Specifically, there have been only two methods used to provide an antenna and RFID tags in a positional relationship that satisfies the conditions of the directivity and the communication distance: either the antenna has been set on the side from which the antenna can face the RFID tags, causing a limitation on the direction of the movement of carts, or several antennas have been set in a plurality of locations around the carts so as to not limit the direction of movement of carts.
Production management and quality management performed by using RFID tags are expected to become more widely used. In particular, UHF (Ultra-High Frequency) band RFID tags allow a communication distance of several meters, which is a greater distance than other types of RFID tags, and by using the UHF band RFID tags it is possible to introduce RFID systems to production lines and inspection lines with a higher degree of freedom in the positional relationship between tags and an antenna for a reader/writer. When introducing the RFID tag systems, it is desirable that limitations on the direction of movement of transfer devices not be imposed and that the number of antennas does not have to be increased.